Stable aqueous dispersions of carbon nanotubes

ABSTRACT

Methods of producing stable dispersions of single-walled carbon nanotube structures in solutions are achieved utilizing dispersal agents. The dispersal agents are effective in substantially solubilizing and dispersing single-walled carbon nanotube structures in aqueous solutions by coating the structures and increasing the surface interaction between the structures and water. Exemplary agents suitable for dispersing nanotube structures in aqueous solutions include synthetic and natural detergents having high surfactant properties, deoxycholates, cyclodextrins, chaotropic salts and ion pairing agents. The dispersed nanotube structures may further be deposited on a suitable surface in isolated and individualized form to facilitate easy characterization and further processing of the structures.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. Provisional PatentApplication Ser. No. 60/303,816, entitled “Isolation and Purification ofSingle Walled Carbon Nanotube Structures” and filed Jul. 10, 2001. Thisapplication is further a divisional application of U.S. patentapplication Ser. No. 09/932,986, entitled “Production of Stable AqueousDispersions of Carbon Nanotubes” and filed Aug. 21, 2001 now U.S. Pat.No. 6,878,361. The disclosure disclosures of the above-mentioned patentapplications are incorporated herein by reference in their entireties.

GOVERNMENT INTERESTS

This invention was made with Government support under contract NCC9-41awarded by the National Aeronautics and Space Administration. TheGovernment has certain rights in the invention.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to methods for isolating and purifyingsingle-walled carbon nanotubes from contaminating materials, such ascarbon and metal catalyst particles, present in the unpurified materialfollowing production of the single-walled carbon nanotube structures.Specifically, the present invention relates to utilizing solutions ofsuitable dispersal agents to isolate and purify individual single-walledcarbon nanotube structures from a raw material including bundles ofnanotube structures.

2. Description of the Related Art

There has been significant interest in the chemical and physicalproperties of carbon nanotube structures since their discovery in 1991,due to the vast number of potential uses of such structures,particularly in the field of nanotechnology, composite materials,electronics and biology. Accordingly, there has been an increase indemand in recent years for carbon nanotube structures for research andapplication purposes, resulting in a desire to produce in an efficientmanner single-walled carbon nanotube (SWCNT) structures that are free ofimpurities and easily separable for their proper characterization.

The three most common manufacturing methods developed for the productionof SWCNT structures are high pressure carbon monoxide (HipCO) processes,pulsed laser vaporization (PLV) processes and arc discharge (ARC)processes. Each of these processes produce SWCNT structures bydepositing free carbon atoms onto a surface at high temperature and/orpressure in the presence of metal catalyst particles. The raw materialformed by these processes includes SWCNT structures formed as bundles oftubes embedded in a matrix of contaminating material composed ofamorphous carbon (i.e., graphene sheets of carbon atoms not formingSWCNT structures), metal catalyst particles, organic impurities andvarious fullerenes depending on the type of process utilized. Thebundles of nanotubes that are formed by these manufacturing methods areextremely difficult to separate.

In order to fully characterize the physical and chemical properties ofthe SWCNT structures formed (e.g., nanotube length, chemicalmodification and surface adhesion), the contaminating matrix surroundingeach structure must be removed and the bundles of tubes separated anddispersed such that each SWCNT structure may be individually analyzed.By maintaining an appropriate dispersal of individual SWCNT structures,characterization of the nanotubes formed may be accomplished in amechanistic manner. For example, it is desirable to easily analyze andcharacterize dispersed SWCNT structures (e.g., determine change innanotube length, tensile strength or incorporation of defined atoms intothe carbon matrix of the SWCNT structure) based upon a modification toone or more elements of a manufacturing method.

It is further highly desirable to produce individual and discrete SWCNTstructures in a form rendering the structures easily manipulable for usein the previously noted fields. At best, existing methodologies capableof physically manipulating discrete material components require elementsthat are measured on micron-level dimensions rather than the nanometerlevel dimensions of conventional partially dispersed and purified SWCNTstructures. However, biological systems routinely manipulate withprecise spatial orientation discrete elements (e.g., proteins) havingphysical dimensions on the order less than SWCNT structures. Thus, ifSWCNT structures could be biologically derived so that biological“tools”, such as immunoglobulins or epitope-specific binding proteins,could be utilized to specifically recognize and physically manipulatethe structures, the possibility of accurately spatially orienting ofSWCNT structures becomes feasible. In order for this approach to berealized, the SWCNT structures must be individually separated from theraw material in a manner consistent with the optimal functioning ofbiological compounds during both the biological SWCNT derivitization andthe manipulation processes. In other words, the SWCNT structures must beproduced as individual, freely dispersed structures in an aqueous buffersystem that exhibits a nearly neutral pH at ambient temperatures inorder to effectively manipulate the structures.

Current methods for purifying and isolating SWCNT structures by removingthe contaminating matrix surrounding the tubes employ a variety ofphysical and chemical treatments. These treatments include hightemperature acid reflux of raw material in an attempt to chemicallydegrade contaminating metal catalyst particles and amorphous carbon, theuse of magnetic separation techniques to remove metal particles, the useof differential centrifugation for separating the SWCNT structures fromthe contaminating material, the use of physical sizing meshes (i.e.,size exclusion columns) to remove contaminating material from the SWCNTstructures and physical disruption of the raw material utilizingsonication. Additionally, techniques have been developed to partiallydisperse SWCNT structures in organic solvents in an attempt to purifyand isolate the structures.

All of the currently available methods are limited for a number ofreasons. Initially, it is noted that current purification methodsprovide a poor yield of purified SWCNT structures from raw material. Afinal SWCNT product obtained from any of these methods will alsotypically contain significant amounts of contaminating matrix material,with the purified SWCNT structures obtained existing as ropes or bundlesof nanotubes thereby making it difficult to analyze and characterize thefinal SWCNT structures that are obtained. These methods furthertypically yield purified SWCNT structures of relatively short lengths(e.g., 150-250 nm) due to the prolonged chemical or physical processingrequired which causes damage to the nanotubes. Additionally, a number ofisolation techniques currently utilized require organic solvents orother noxious compounds which create environmental conditions unsuitablefor biological derivitization of SWCNT structures. Organic solventscurrently utilized are capable of solubilizing SWCNT structures inbundles and not individual, discrete tubes. Furthermore, presentisolation techniques require prolonged periods of ultra-speedcentrifugation (i.e., above 100,000×g) in order to harvest nanotubestructures from solvents or other noxious compounds used to removecontaminating matrix material from the nanotubes.

Presently, the overwhelming problem for industrial and academiclaboratories engaged in the use of carbon nanotubes for research as wellas other applications is the limited source of discrete, completelyseparated SWCNT structures. Investigations into the vast potential ofuses for SWCNT structures are being hampered by the limited supply ofwell characterized SWCNT material free of significant amounts ofcontaminants like amorphous carbon and metal catalyst particles.

Accordingly, there presently exists a need for harvesting high yields ofpurified SWCNT structures from the raw material of a carbon nanotubeproduction process in a fast and efficient manner to meet the demand forsuch structures. Additionally, it is desirable to provide SWCNTstructures as discrete and individual structures (i.e., not bundledtogether), having suitable lengths and well characterized for biologicalderivitization and easy manipulation.

SUMMARY OF THE INVENTION

Therefore, in light of the above, and for other reasons that will becomeapparent when the invention is fully described, an object of the presentinvention is to provide a rapid and effective method of isolating andpurifying SWCNT structures disposed within a raw material containingcontaminants to obtain a high product yield of quality SWCNT structureshaving appropriate lengths suitable for different applications.

Another object of the present invention is to provide a method ofdispersing isolated and purified SWCNT structures in solution from theraw material so as to yield discrete and separated nanotube structuressuitable for different applications.

A further object of the present invention is to provide a method ofdispersing isolated and purified SWCNT structures in a suitable solutionto render the structures suitable for biological derivitizationprocedures to effect easy manipulation of the SWCNT structures.

A further object of the present invention is to produce an aqueousdispersion of single, discrete SWCNT's that remains stable over aprolonged period of time (i.e. weeks to months), a dispersion in whichaggregation or “flocking” of SWCNT's does not occur.

The aforesaid objects are achieved in the present invention, alone andin combination, by providing a method of dispersing a matrix of rawmaterial including SWCNT structures and contaminants in an aqueoussolution containing a suitable dispersal agent to separate theindividual SWCNT structures from the matrix, thus purifying anddispersing the structures within the solution. In solution, thedispersal agent surrounds and coats the individual SWCNT structures,allowing the structures to maintain their separation rather thanbundling together upon separation of the structures from solution.Suitable dispersal agents useful in practicing the present invention aretypically reagents exhibiting the ability to interact with hydrophobiccompounds while conferring water solubility. Exemplary dispersal agentsthat can be used in the present invention include synthetic and naturaldetergents, deoxycholates, cyclodextrins, poloxamers, sapogeninglycosides, chaotropic salts and ion pairing agents.

The above and still further objects, features and advantages of thepresent invention will become apparent upon consideration of thefollowing detailed description of specific embodiments thereof,particularly when taken in conjunction with the accompanying drawingswherein like reference numerals in the various figures are utilized todesignate like components.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a is plot of % transmission (% T) values for aqueous solutionscontaining three synthetic detergents having varying surfactantstrengths.

FIG. 1 b is a plot of % transmission (% T) vs. time for the aqueoussolutions of FIG. 2, wherein the solutions have undergone evaporation.

FIG. 1 c is a plot of % transmission (% T) vs. time for aqueoussolutions of FIG. 2, wherein the solutions have undergone noevaporation.

FIG. 2 is a plot of % transmission (% T) vs. time for aqueous solutionscontaining taurocholic acid, Poloxamer 188, saponin andmethyl-β-cyclodextrin.

FIG. 3 is a plot of % Transmission values vs. fraction #'s measuredduring fractionation of methyl-β-cyclodextrin dispersed SWCNT structuresin a 5000 MW size exclusion column.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to a method for purifying and isolatingSWCNT structures from raw material by dispersing the structures in anaqueous solution with a biologically active dispersal agent. Thebiologically active dispersal agent effects a separation of the SWCNTstructures from contaminating material such that the purified SWCNTstructures exist as a dispersion of individual and discrete SWCNTstructures in solution. As used herein, the term “raw material” refersto material formed by any process for producing single-walled carbonnanotubes, including, without limitation, the three processes describedabove. The raw material typically contains SWCNT structures embedded ina matrix of contaminating material. The terms “contaminating material”and “contaminants”, as used herein, refer to any impurities or othernon-SWCNT components in the raw material including, without limitation,amorphous carbon and metal catalyst particles.

As previously noted, the current methods employed for purifying andharvesting SWCNT structures have met with limited success due in part tothe traditional view of SWCNT structures as chemical compounds. In adeparture from the traditional view, SWCNT structures are consideredhere as being similar to biologically derived structures. Some notedproperties of SWCNT structures are as follows (not all of which must bepresent): they are typically insoluble in water; they typically selfassociate as bundles or ropes; they are made exclusively of carbon; andeach end of a carbon nanotube will typically exhibit differentphysiochemical properties. The physical properties of carbon nanotubesare in fact very similar to lipids, which are a class of biologicalcompounds insoluble in water but capable of being solubilized in aqueoussolutions including suitable lipid dispersing reagents. As such, theinventors recognized that SWCNT structures are readily dispersablewithin an aqueous solution containing a reagent typically suitable fordispersing proteins or lipids in aqueous solutions.

Reagents considered effective in suitably dispersing SWCNT structures inaqueous solution are referred to as dispersal agents. A dispersal agentcan be any suitable reagent that is effective in substantiallysolubilizing and dispersing SWCNT structures in an aqueous solution byincreasing the interaction at the surface interface between eachnanotube structure and water molecules in solution. The underlyingmechanisms whereby a dispersal agent brings about dispersion ofindividual SWCNT structures, from the “bundles” or “ropes” in which theyare constitutively formed, into an aqueous solution is primarily basedupon the ability of the dispersal agent to break down the molecularforces at the surface of the SWCNT preventing water molecules frominteracting with the SWCNT surface. In addition to this property, due tothe large surface area of the SWCNT, it also essential that thedispersal agent have a molecular structure that maximizes its ability toreduce hydrophobic interactions between individual SWCNT's, while alsobeing of a small enough size to easily penetrate into the inter-SWCNTspaces. A further requirement of an efficient SWCNT dispersal agent isthat it also can remain in aqueous solution at a high enoughconcentration so that a useful dispersal agent concentration ismaintained for SWCNT “bundle” or “rope” dispersal, even after a portionof the original amount in solution has been utilized for the dispersionof non-SWCNT contaminants in the raw nanotube material. The dispersalagent is typically added to an aqueous solution in an effective amountto substantially purify and disperse SWCNT structures in solution. Theeffective amount of dispersal agent will vary based upon the type ofdispersal agent utilized in a particular application.

The dispersal agents are typically synthetic or naturally occurringdetergents or any other composition capable of encapsulating andsuitably solubilizing hydrophobic compounds in aqueous solutions.Exemplary dispersal agents include, without limitation, synthetic ornaturally occurring detergents having high surfactant activites such asNONIDET™ P-40 or (NP-40), polyoxyethylene sorbitol esters (e.g., TWEEN®and EMASOL™ series detergents), poloxamers (e.g., the Pluronic series ofdetergents and Poloxamer 188), and ammonium bromides and chlorides(e.g., cetyltrimethylammonium bromide, tetradecylammonium bromide anddoddecylpyridinium chloride), naturally occurring emulsifying agentssuch as deoxycholates and deoxycholate-type detergents (e.g.,taurocholic acid), sapogenin glycosides (e.g., saponin) andcyclodextrins (e.g., α-, β- or γ-cyclodextrin), chaotropic salts such asurea and guanidine, and ion pairing agents such as sulfonic acids (e.g.,1-heptane-sulfonic acid and 1-octane-sulfonic acid).

Naturally occurring emulsifying agents such as taurocholic acid andcyclodextrins are highly effective in solubilizing and dispersing SWCNTstructures and in facilitating biological derivitization of the purifiedand isolated SWCNT structures. In particular, cyclodextrins have a threedimensional doughnut shaped orientation with a “torsional” structurecomposed of glucopyranose units. The “torsional” structure of acyclodextrin molecule allows it to attract and interact with the surfaceof a SWCNT structure within its central hydrophobic region, even whenphysically altered from a round “doughnut” shape to a twisted “doughnut”shape, while maintaining an outer hydrophilic surface rendering themolecule soluble in aqueous solutions. The solubility of nativecyclodextrins in water may also be increased nearly tenfold bysubstitution of, e.g., methyl or hydroxypropyl groups on thecyclodextrin molecule. Greater solubility of the cyclodextrin in watertranslates to a greater dispersion and isolation of individual SWCNTstructures in solution. Two exemplary cyclodextrin derivatives that arehighly effective in dispersing SWCNT structures in solution aremethyl-β-cyclodextrin (MβC) and 2-hydroxypropyl-β-cyclodextrin(2-HP-β-C). However, it is noted that any cyclodextrin (i.e., α, β orγ), or any suitable derivative thereof, may be utilized in accordancewith the present invention. Further, cyclodextrins are useful forbiological derivitization of SWCNT structures which have been isolatedin solution. Taurocholic acid (TA), which is exemplary of a suitabledeoxycholate-type detergent capable of substantially dispersing SWCNTstructures in solution, is produced naturally in mammalian liver tissue.It is also highly effective in facilitating biological derivitization ofpurified and isolated SWCNT structures because, like the cyclodextrins,TA has a molecular shape that allows a large surface area of SWCNTstructures to be coated per molecule of TA. Typically, cyclodextrins anddeoxycholates may be utilized to suitably disperse SWCNT structuresaccording to the present invention in concentrations ranging from about5 mg/ml to about 500 mg/ml of aqueous solution, with a preferableconcentration of about 50 mg/ml.

Sapogenin glycosides (e.g. saponin), another naturally occurring classof emulsifying agent of plant origin, are also capable of dispersingSWCNT structures. Like both the cyclodextrins and deoxycholate typedetergents, these compounds are amphiphilic in nature, have a highsolubility in water and can act as protective colloids to normally waterinsoluble hydrophobic compounds (i.e., a SWCNT structure) in an aqueoussolution. Solubilization of raw SWCNT material has been achieved atconcentrations between 0.1 mg/ml up to 50 mg/ml of aqueous solution inthe present invention, with a preferable concentration of about 10mg/ml.

Synthetic detergents suitable for use as dispersal agents here willtypically have a high surfactant activity and be utilized in amounts ofabout 50-95% of their critical micelle concentration (CMC) values. Thesehigh surfactant detergents are capable of overcoming hydrophobic forcesat the SWCNT surface/aqueous solution interface by coating the SWCNTstructures to establish suitable solubility of the SWCNT structures insolution. The surfactant properties of a synthetic detergent may becharacterized in terms of a hydrophilic-lipophilic balance (HLB), whichprovides a measurement of the amount of hydrophilic groups tohydrophobic groups present in a detergent molecule. In particular,synthetic detergents that are suitable for use as dispersal agents herehave an HLB value between about 7 and about 13.2. Limiting theconcentration of the synthetic detergent to a suitable level below itsCMC will also ensure adequate dispersion of the SWCNT structures withoutthe formation of floccular material due to self-association of thedetergent molecules. Additionally, chaotropic salts (e.g., urea andguanidine) are typically utilized as dispersal agents in concentrationsranging from about 6M to about 9M in solution (wherein “M” refers tomolarity), whereas ion pairing agents are typically utilized asdispersal agents in concentrations ranging from about 1 mM to about 100mM in solution.

While selection of a suitable dispersal agent as well as a suitableconcentration is important for achieving a desirable dispersion of SWCNTstructures in aqueous solution, other factors may also enhance thedispersing effect of the dispersal agent. Exemplary factors that affectdispersion of SWCNT structures in aqueous solutions include, withoutlimitation, the pH of the solution, cation concentration (e.g., sodium,potassium and magnesium) in solution, and other conditions such asoperating temperature and pressure. Indeed, due to the unique propertiesof the solvent in this case, namely water, specifically the uniquechemical interactions that can exist between individual water molecules(i.e. hydrogen bonding, molecular aggregation) it is predicted thatdecreasing the operating temperature (rather than increasing thetemperature as is the case in most common chemical reactions) willincrease the ability of a dispersal agent to disperse SWCNT material dueto a reduction in hydrophobic interactions between the SWCNT surface andthe water molecules at lower operating temperatures (i.e. between 0° C.and 10° C.). By interacting with the surface of individual SWCNTstructures, the dispersal agent molecules typically surround and coatthe exposed hydrophobic surface of the SWCNT. This interaction resultsin the separation of individual SWCNT structures from the bundles inwhich they were formed and from contaminating matrix material, and dueto the amphiphilic nature of the dispersal agent maintains the nowdiscretely separated, individual SWCNT structures in the form of anaqueous dispersion or colloidal solution.

The raw material containing SWCNT structures typically is added to anaqueous solution containing the dispersal agent and appropriately mixed(e.g., by mechanical agitation or blending) to ensure adequateinteraction and coating of dispersal agent molecules with SWCNTstructures. While the amount of SWCNT material that may be added to anaqueous dispersal agent solution to obtain an effective dispersion ofSWCNT structures typically depends upon factors such as the specificdispersal agent utilized and its concentration in solution, effectivedispersions have been achieved utilizing concentrations as high as 1mg/ml of SWCNT structures in aqueous dispersal agent solution. Uponadequate mixing, the solution containing the dispersed SWCNT structuresmay be filtered with an appropriately sized filter (e.g., about 0.05-0.2μm filtration) to remove any insoluble material (e.g., matrixcontaminants) remaining in solution. Typically, a 0.2 μm filter isutilized to ensure adequate removal of contaminants while preventingcaking of the filter and loss of dispersed SWCNT structures. However,smaller pore size filters may also be utilized to ensure more efficientremoval of contaminants. In situations where a smaller pore size filteris implemented, any SWCNT structures that may have become trapped in thefilter cake may be recovered by resuspension of the cake in dispersalagent solution and repeating filtration steps as necessary to obtain adesirable yield. Alternatively, a standard cross-flow filtration systemcan be utilized to reduce the amount of caking that occurs on thesurface of the filter.

Additional processing steps, such as centrifugation or other separationtechniques, may also be utilized to remove insoluble material and excessdispersal agent from solution after the SWCNT structures have beensuitably dispersed therein. Specifically, the SWCNT structures may bewashed to remove excess dispersal agent by subjecting the solution tocentrifugation at speeds ranging from about 100×g to about 10,000×g tosediment SWCNT structures. The SWCNT structures may then be removed fromsolution and re-dispersed in distilled water. The washing process may berepeated any desired number of times to ensure adequate removal ofexcess dispersal agent. The SWCNT structures may also be separated fromexcess dispersal agent and other contaminants in solution via dialysisor the use of an appropriate size exclusion column (e.g., a 5000 MW sizeexclusion column). The resultant solution, which contains substantiallyisolated and purified SWCNT structures coated with dispersal agent, ishighly useful in a variety of applications, particularly nanotechnologyresearch. As for example in the case of protein biochemistry, whereprotein function may be negatively impacted by the use of a particulardispersal agent as a prerequisite to enable the extraction of theprotein from contaminating matrix, once dispersal of the protein into anaqueous solution has been achieved, the offending dispersal agent can besubstituted with a second dispersal agent. This second dispersal agentalso maintains the protein molecule in aqueous solution but is moreappropriate for those applications where protein function is paramount.In a similar fashion, once dispersal of SWCNT material has beenachieved, if necessary the primary dispersal agent can be substitutedwith a second dispersal agent that is more suitable for the envisioneduse. For example, a cyclodextrin can be used as the primary dispersalagent to produce a stable dispersion of individual SWCNT's in an aqueoussolution. The cyclodextrin dispersal agent can then be substituted with,for example, a secondary dispersal agent such as Poloxamer 188. Thesecondary dispersal agent provides a matrix material that coats thesurface of individual SWCNT's that is easily polymerized to form, forexample, an SWCNT-containing composite material, or a readily accessiblesource of chemical groups now associated with the surface of thedispersed SWCNT's that can be easily modified using standardchemistries.

One central aim of the present invention is to ensure that SWCNT's aredispersed in an aqueous solution in the form of individual, discretenanotubes (i.e., a complete dispersion). One characteristic of SWCNTmaterial dispersed in an aqueous solution that contains SWCNT's, thatremain either in non-dispersed bundles or that have been individuallyseparated but have re-associated into bundles (i.e., an incompletedispersion), is that the SWCNT material re-associates into largeaggregates or “flocs” which in turn sediment out of solution. Thisprocess occurs within a matter of minutes to hours, even after filteringthe dispersion through a 0.2 micron filter. In the case of a completedispersion, filtering the solution through a 0.2 micron filter resultsin a colored liquid that essentially does not flock or aggregate overtime and remains stable (i.e., no flocking) for extended periods oftime. As long as the relative concentration of the dispersed SWCNT's ordispersal agent in the aqueous solution is not increased by evaporationof water the dispersion remains stable (i.e., SWCNT's remain as singlediscrete nanotubes in the aqueous solution).

The maximum concentration of SWCNT material that can be maintained insolution by a particular dispersal agent is related to the total surfacearea of the individual SWCNT's present in the solution. A completeaqueous dispersion of SWCNT material exists where there is a balancebetween the amount of SWCNT surface area exposed to the solvent (i.e.,water) and the amount of dispersal agent available to interact with thatexposed SWCNT surface in order to confer water solubility on the SWCNT.The amount of dispersal agent available to interact with the exposedSWCNT surface in aqueous solution is in turn dependent on the watersolubility of the particular dispersal agent and the amount of SWCNTsurface great that each individual dispersal agent molecule is capableof interacting with. As such, there is a maximal amount of SWCNTmaterial that can be completely dispersed at a particular concentrationof a particular dispersal agent. The maximal amount of SWCNT materialthat can be dispersed using a particular dispersal agent is achieved ator below the concentration of dispersal agent in aqueous solution whereself-association of the dispersal agent occurs (known as the CMC in thecase of detergents and Cloud Point in the case of emulsifying agents).In addition, the amount of SWCNT material that can be dispersed in sucha dispersal agent solution cannot however exceed the saturationconcentration of individual, discrete SWCNT'S in solution, which due totheir large physical size exhibit colloidal properties. This maximumSWCNT concentration is in turn dependent on the length of the SWCNT(i.e., its molecular size), the longer the SWCNT the lower the maximalconcentration that can be maintained as a complete dispersion in anaqueous solution at a constant dispersal agent concentration. Based onthis understanding, for a particular dispersal agent dissolved at itsoptimal concentration in water, there is also a maximum concentration(i.e., number) of SWCNT's that can be maintained as a complete aqueousdispersion, where that maximum number is directly related to the surfacearea of the SWCNT's in solution.

For example, a SWCNT of 100 nm in length and 1 nm in diameter has anexposed external surface area of 100 πnm². A SWCNT of 10,000 nm (e.g. 10microns) in length and 1 nm in diameter has an exposed external surfacearea of 10,000 πnm². As such, it requires the same amount of dispersalagent to maintain one hundred, 100 nm long SWCNT's as a completedispersion as it does to keep a single 10 micron SWCNT in completedispersion. This example demonstrates the importance of the relationshipbetween (1) molecular shape of the dispersal agent (i.e., the amount ofSWCNT surface area that a single molecule of dispersal agent caninteract with), (2) concentration of the dispersal agent in solution,(3) overall exposed SWCNT surface area (related to SWCNT length) and (4)the maximal amount of SWCNT material that can exist as a completeaqueous dispersion.

The following examples disclose specific methods for isolating andpurifying SWCNT structures from raw material containing contaminants.Specifically, NP-40, TA, Poloxamer 188, saponin and a cyclodextinderivative are utilized to show the effect of each in dispersing SWCNTstructures in aqueous solution. The raw material containing SWCNTstructures for each example was obtained utilizing a PLV process.However, it is noted that the SWCNT structures may be isolated andpurified utilizing raw material provided via any process. It is furthernoted that the examples are for illustrative purposes only and in no waylimit the methods and range of dispersal agents contemplated by thepresent invention.

Example 1

Raw material containing bundled SWCNT structures was mixed into threesynthetic detergent solutions known for solubilizing proteins and lipidsin aqueous solutions. The three synthetic detergents utilized wereNP-40, SDS and TX-100. These detergents were selected due to theirdiffering physical properties and to demonstrate how the surfactantactivity of the detergent affects the dispersion of SWCNT structures insolution. SDS is a strong anionic detergent that solubilizes compoundsin water by virtue of coating the compounds with a layer ofnegatively-charged, water soluble detergent molecules. In contrast, bothTX-100 and NP-40 are non-ionic detergents that function via hydrophobicinteractions with the surface of a compound, thereby forming a watersoluble layer of detergent molecules around the water insolublecompound. The surfactant properties (i.e. ability to decrease surfacetension between aqueous and non-aqueous phases) for NP-40 are muchgreater than SDS and TX-100. Reported HLB values for each of thesedetergents are as follows (e.g., see Kagawa, Biochim. Biophys. Acta 265:297-338 (1972) and Helenius et al., Biochim. Biophys. Acta 415: 29-79(1975)):

Detergent HLB SDS 40 TX-100 13.5 NP-40 13.1

Three aqueous solutions were each prepared as follows. A 1 mg (total dryweight) amount of raw material was solubilized in 1 ml of doubleglass-distilled, deionized water (ddH₂O) containing one of thedetergents (e.g., SDS, TX-100 or NP-40) at 50% of its respective CMCvalue. Each solution was subsequently vortexed for 30 minutes at roomtemperature. The resultant dispersions were passed through a 0.2 μmcellulose acetate filter to remove any particulate matter. Conventionalspectroscopy methods were employed to measure the percent transmission(% T) of each solution at a wavelength of 450 nm (path length of 3 mm).

The % T value of each the solutions was measured to provide anindication of solution color and thus comparatively determine theability of each detergent to effectively disperse SWCNT structureswithin solution. Specifically, % T values are inversely proportional tothe degree of color in solution. If SWCNT structures are bundledtogether in a particular solution (or begin to re-aggregate intobundles), flocular material forms removing SWCNT structures fromsolution by sedimentation, thus decreasing the color and increasing the% T value over time. Alternatively, if SWCNT structures remain dispersedin solution, no flocking occurs and the color solution remainsconsistent. Therefore, a lower % T value measured in the filtrate wouldindicate a higher level of dispersion of SWCNT material in solution, anda constant % T over time reflects a stable SWCNT dispersion.

The plots illustrated in FIGS. 1 a-1 c provide % T data for solutionscontaining SDS, TX-100 and NP-40, respectively, with and without SWCNTstructures. The unshaded bar portions in FIG. 1 a represent % T valuesmeasured for each detergent solution absent any raw material. The % Tvalue for the shaded bar portions represent % T values measured for eachdetergent solution containing SWCNT structures at a time shortly after0.2 μm filtration of the solution. The shaded bar data of FIG. 2 clearlyindicates that NP-40, which has the greatest surfactant properties, hasa much lower % T value than both SDS and TX-100 and thus provides asubstantially more effective dispersion of SWCNT structures in aqueoussolution.

To illustrate the effect of detergent concentration on SWCNT dispersionin solution, the solutions containing SWCNT structures were allowed toevaporate from an initial volume of 150 μl to a final volume of 50 μlover a period of 16 hours at room temperature. Intermittent % Tmeasurements were taken, and the results are illustrated in FIG. 1 b.The % T values for each solution containing a detergent and SWCNTstructures increased with time (i.e., correlating with a decrease incolor), which coincided with a noticeable appearance of flocularmaterial in the detergent dispersions thus indicating that nanotubeswere beginning to re-associate into larger bundles that were insolublein water. The test results indicate that, as the detergent concentrationincreases above its CMC value, micelle formations occur in solutionresulting in reduced dispersion of the SWCNT structures. Thus, selectionof detergent concentration is very important in maintaining dispersionof the SWCNT structures in solution. Alternatively, the results (FIG. 1b) could indicate that as the volume of the solution decreased due towater evaporation, not only did the relative concentration of thedetergent increase above its CMC resulting in a functional decrease inthe amount of dispersal agent available to maintain discrete individualSWCNT's in solution, but so too did the relative concentration of theSWCNT's in the solution. Due to the colloidal nature of the SWCNTdispersion, this process could in isolation, or, in conjunction with thedetergent concentration rising above the CMC, result in re-aggregationor “flocking” of SWCNT's in the dispersion, an event that in turn isreflected by an increase in % T.

A further test was conducted with solutions prepared in a substantiallysimilar manner as the previous solutions. However, these solutions werestored in sealed vials at room temperature so as to prevent theirevaporation. As indicated by the data depicted in FIG. 1 c, there wasrelatively no change in % T value for each of the different detergentsolutions and no noticeable appearance of flocular material after a 72hour period, or an increase in % T after a second round of filtrationthrough a 0.2 μm filler.

The data of example 1 indicates that a strong surfactant such as NP-40is highly effective in dispersing SWCNT structures in aqueous solutionswhen utilized in an effective amount. Further, NP-40 can maintain asuitable dispersion of the structures in solution for extended periodsof time. Weaker surfactants having HLB values greater than 13.2, such asSDS and TX-100, may provide some dispersion but will not be effective insubstantially isolating and purifying SWCNT structures from rawmaterial.

Example 2

Aqueous solutions of each of the TA, Poloxamer 188, saponin and MβC wereprepared alone and with raw material as follows. Specifically, eachsolution was prepared by solubilizing 1 mg of the raw material in 1 mlof ddH₂O containing either 50 mg/ml of TA, 50 mg/ml of MβC, 10 mg/ml ofsaponin or 2% (v/v) Poloxamer 188. Each resultant solution was vortexedfor 30 minutes at room temperature and then filtered through a 0.2 μmcellulose acetate filter. The % T values were measured for the filtratesat room temperature immediately after filtration, 72 hr after storage ina sealed vial of the original filtered solutions and again immediatelyafter a second filtration of the stored solutions. Dispersalagent/SWCNT-containing solutions were compared to aqueous solutionscontaining dispersal agent alone treated in an identical fashion.

The % T values illustrated in FIG. 2 reveals that the SWCNT structuresremained dispersed in the TA, Poloxamer 188, saponin and MβC filtratesfor the entire 72 hour period, as is evident from the relativelyconstant % T values measured for each filtrate over that time period. Asdemonstrated in FIG. 1 b, if flocking or re-aggregation of dispersedSWCNT material occurs, this results in a decrease in the % T of thedispersion. In addition, if flocking or re-aggregation of SWCNT materialin the dispersion (FIG. 2) had occurred to any extent during the 72 hourperiod of storage without water evaporation, filtration of thisdispersion through a 0.2 μm filter for a second time will result in theremoval of this re-aggregated or flocked material resulting in adecrease in % T value of the dispersion. The data further indicates MβCfiltrates had considerably lower % T values, correlating to a greaterdispersion of SWCNT structures, than either the TA, the Poloxamer 188,or saponin filtrates and the NP-40 filtrate of FIG. 1 c. No significantincreases in % T values were observed even after a second round of 0.2μm filtration of each filtrate after the 72 hour period. The resultsprovided in FIG. 2 clearly indicate that TA, saponin, Poloxamer 188 andMβC serve as highly effective dispersal agents, providing substantialdispersions of the SWCNT structures in aqueous solution for extendedperiods of time.

Example 3

SWCNT structures dispersed in the TA and MβC solutions of the previousexample were separated from the impurities in solution bycentrifugation. Specifically, SWCNT structures sedimented out of a 1 mlvolume of either solution having a liquid column height of 2.5 cm at acentrifugation speed of 10,000×g. In addition, sub-populations ofSWCNT's dispersed in either NP-40, TA, MβC, saponin or Poloxamer 188 canbe collected from the same sample by using sequentially increasingcentrifugation speeds (e.g. 1,000×g, 2,500×g, 5,000×g and 7,500×g). Assuch, these results suggest that as is the case in biologicalseparations where differential centrifugation can be used to separatecellular structures based upon their size (e.g., see Techniques Reviewedin Subcellular Fractionation—A practical approach; edited by J M. Grahamand D. Rickwood, IRL Press, Oxford, 1996), a similar approach can beutilized to collect SWCNT's of different sizes from the aqueousdispersions described here. It is noted that prior SWCNT purificationtechniques typically require centrifugation speeds in excess of100,000×g to yield any sedimentation of SWCNT structures, anexperimental observation that is consistent with the presence of veryshort (i.e., less than about 250 nm) SWCNT's being present in suchprevious solutions. The ability to sediment SWCNT's from the aqueousdispersions described in the present invention at the relatively lowg-forces indicated above (i.e., below 10,000×g) indicates that the SWCNTstructures present in those dispersions must be much larger (i.e.,longer as the diameter of a SWCNT is a constant dimension ofapproximately 1 nm) than those previously produced and that requireabove 10,000×g forces to bring about sedimentation.

Example 4

Aqueous MβC solutions containing dispersed SWCNT structures wereprepared as follows. Two hundred μg of SWCNT containing raw material wassolubilized in a 1 ml solution of ddH₂₀ containing 50 mg/ml of MβC. Thesolution was physically homogenized in a miniaturized inversion blenderat about 23,000 RPM. The resultant dispersion was subsequently vortexedfor 30 minutes at room temperature followed by 100×g centrifugation for10 minutes to sediment any remaining insoluble material. The resultantsupernatant was then passed through a 5000 MW cut-off gravity-fed sizeexclusion column (10 ml bed volume) in the following manner. One ml ofthe dispersed solution was placed on the top of the column, which hadbeen conditioned with 50 ml of ddH₂O. One ml fractions were thencollected from the base of the column as ddH₂O was added to the top ofthe column. The % T values were measured for each collected fraction. Aplot of the % T values vs. fraction# is illustrated in FIG. 3. Coloredfractions, as indicated by the decreasing % T values, were indicative ofdispersions in solution. Those colored fractions (i.e., fraction#'s 1-10of FIG. 3) were collected and pooled together. This procedure wasconducted to remove excess MβC from the SWCNT dispersions. The resultantsolution containing the dispersed SWCNT structures was centrifuged at10,000×g to sediment SWCNT structures from solution. The supernatant wasthen discarded and the sedimented SWCNT's were resuspended in distilledwater to form a stable aqueous dispersion of discrete separated SWCNT'scontaining little or no excess of dispersal agent. Again, an approachthat has commonly been used in biological science to achieve separationof biological molecules, namely size exclusion chromatography, can besuccessfully applied to the experimental problems encountered in theseparation and purification of SWCNT material. Based upon anunderstanding of biological separation techniques, the elution ofdifferent amounts of dispersed SWCNT's from a size exclusion columnafter different retention times (as indicated by differing % T values ineach of the fractions, i.e., Fraction #1-10, in FIG. 3) suggests thatdiscrete SWCNT's truly dispersed in an aqueous solution can be separatedand purified on the basis of their length using size exclusionchromatography in a similar fashion to that employed for separating andpurifying proteins of different sizes. In conjunction with the previousExamples 1-3, these data strongly suggest that the approach described inthe present invention to solving the experimental problems encounteredin the separation and purification of SWCNT material, based upon thenovel and innovative concept that the SWCNT structures behaveessentially as “biological” compounds, rather than as a product ofphysical or organic chemistry, has been successful.

Having described novel methods of producing stable aqueous dispersionsof SWCNT's and corresponding products thereof, it is believed that othermodifications, variations and changes will be suggested to those skilledin the art in view of the teachings set forth herein. It is therefore tobe understood that all such variations, modifications and changes arebelieved to fall within the scope of the present invention as defined bythe appended claims.

1. A single-walled carbon nanotube product comprising a solutionincluding single-walled carbon nanotube structures coated with adispersal agent, wherein said dispersal agent is selected from the groupconsisting of detergents selected from the group consisting ofpolyoxyethylene sorbitol esters, poloxamers; saponin glycosides;deoxycholates; chaotropic salts and ion pairing agents, and wherein saidstructures are substantially dispersed within said solution.
 2. Asingle-walled carbon nanotube product comprising a stable, aqueoussolution including single-walled carbon nanotube structures coated witha dispersal agent, wherein the single-walled carbon nanotube structurescomprise single-walled carbon nanotube surfaces and wherein thedispersal agent coats the surfaces of the single-walled carbonnanotubes, wherein said dispersal agent is selected from the groupconsisting of poloxamers, saponin glycosides, deoxycholates,polyoxyethylene sorbitol esters, ammonium bromides and ammoniumchlorides, taurocholic acid, chaotropic salts and ion pairing agents,and wherein said structures are substantially dispersed within saidaqueous solution.
 3. The product of claim 2, wherein said structures arefurther substantially free of contaminants.
 4. The product of claim 2,wherein said dispersal agent comprises a detergent in said solution inan amount at least about 50% of a critical micelle concentration of saiddetergent.
 5. The product of claim 2, wherein said dispersal agent is adetergent selected from the group consisting of polyoxyethylene sorbitolesters, ammonium bromides and ammonium chlorides.
 6. The product ofclaim 2, wherein said dispersal agent comprises a detergent in saidsolution in an amount no greater than about 95% of a critical micelleconcentration of said detergent.
 7. The product of claim 6, wherein saiddispersal agent comprises a detergent in said solution in an amount atleast about 50% of a critical micelle concentration of said detergent.8. The product of claim 2, wherein said dispersal agent comprises adetergent having a hydrophilic-lipophilic balance no greater than about13.2.
 9. The product of claim 8, wherein said dispersal agent comprisesa detergent having a hydrophilic-lipophilic balance (HLB) of betweenabout 7 and 13.2.
 10. The product of claim 2, wherein said dispersalagent is a chaotropic salt selected from the group consisting of ureaand guanidine.
 11. The product of claim 10, wherein said chaotropic saltis in said solution in an amount no greater than about 9M.
 12. Theproduct of claim 10, wherein said chaotropic salt is in said solution inan amount at least about 6M.
 13. A single-walled carbon nanotube productcomprising a solution including single-walled carbon nanotube structurescoated with a dispersal agent, wherein said dispersal agent is acyclodextrin derivative selected from the group consisting ofmethyl-B-cyclodextrin and 2-hydroxypropyl-B-cyclodextrin, and whereinsaid structures are substantially dispersed within said solution.
 14. Asingle-walled carbon nanotube product comprising a solution includingsingle-walled carbon nanotube structures coated with a dispersal agent,wherein said dispersal agent is selected from the group consisting ofsaponin, and taurocholic acid.
 15. The product of claim 14, wherein saiddispersal agent is in said solution in an amount no greater than about500 mg/ml.
 16. The product of claim 14, wherein said dispersal agent isin said solution in an amount at least about 5 mg/ml.